Conventional color TV signals such as a luminance signal Y, a chrominance subcarrier signal (hereinafter referred to as chrominance signal) C, and a synchronization signal are bundled in a composite color image signal (hereinafter referred to as composite signal) N. The luminance signal Y and the chrominance signal C contained in the composite signal are separated in each TV set. However, perfect separation of the luminance signal Y from the chrominance signal C is not easy, which therefore has been a major source of degradation of the image associated with them.
One approach to circumvent this problem in a DAC for a high-quality video signal processing in a digital TV for example is to provide two separate channels, one for the luminance signal Y and another for the chrominance signal C constituting a color TV signal, in addition to a channel for the conventional composite signal N, thereby forming a three-channel system, as is done in high-resolution VTR for S-image signal.
Chrominance signal C itself is a composite 2-color signal made up of a color difference signal R-Y for red (hereinafter referred to as signal U) and a color difference signal B-Y for blue (hereinafter referred to as signal V). The first color difference signal U, the second color difference signal V, and the luminance signal Y are often used as 3-channel color TV signals to improve the resolution of color TV signals.
Therefore, it is desirable for a semiconductor device having DACs in its video signal processing system that the DACs can be used in such high-resolution processing and in conventional processing as well. Thus, in recent years, 6 DACs are often provided in two separate groups with one group including three channels for luminance signal Y, composite signal N, and chrominance signal C, and another group including three channels for luminance signal Y, first color difference signal U, and second color difference signal V.
This allows a TV user to choose either group of channels in accordance to his preference.
However, such multi-channel system as mentioned above suffers cross talks between the channels, which causes degradation of picture quality, as discussed in detail below.
We first take a look at the first group of three channels, in which the chrominance signal C is used in the subcarrier wave (having 3.58 MHz in NTSC system, or 4.43 MHz in PAL system) to perform orthogonal phase modulation of the first color difference signal B-Y with the second color difference signal R-Y. Hence, energy density is high in the neighborhood of the subcarrier frequency. Then, parasitic capacitors Cp will be created between neighboring channels, since the three channels of DACs are integrated into an IC chip. Therefore, although the chrominance signal C and the luminance signal Y are separated, a cross talk inevitably arises between the luminance signal Y and the chrominance signal C on account of the parasitic capacitors Cp. Further, since the composite signal N in the other channel also includes a chrominance signal, the luminance signal Y is influenced by the cross talk with the chrominance signal in the composite signal N.
In this manner, luminance signal Y is influenced by the high-frequency components of the chrominance signals C in the two parallel neighboring channels, which results in additional cross talks that further degrades the picture quality.
In some cases, multiple loads are driven at the same time. For example, a CRT monitor and a video deck are used simultaneously, or multiplicity of monitors are simultaneously used. In such cases, the color TV signals consisting of the luminance signal Y and the chrominance signal C of the first three channels and the color TV signals in the composite signal N are simultaneously used, or the six channels in the first and the second groups are simultaneously used.
In these circumstances, a semiconductor device having multiple DAC channels must supply larger output currents to the multiple channels than normally expected. Furthermore, these currents change simultaneously that the power source of the IC chip suffers large current/voltage fluctuations, which can be also a source of degradation of picture quality.